net/fec: make FEC driver buildable as module

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>